The manufacture of fibrous composite pressure vessels and rocket motor cases is accomplished by filament winding or by hand lay-up of the fibrous material impregnated with a synthetic resin on a mandrel or core which forms the internal configuration of the finished part. The mandrel, after the synthetic resin component of the fibrous composite is cured, must be removed. On tubular parts, such as fibrous composite pipe and similar structures, removal of the mandrel is not a particularly difficult procedure. However, on pressure vessels and rocket motor cases with restricted access due to the relatively small access ports common in such designs, removal of the mandrel from the finished part is a major design consideration for both the pressure vessel design and the mandrel design.
Other important attributes desired in mandrels for filament wound fibrous composite structures are light weight, stiffness, dimensional stability and low life cycle and/or recurring costs. Weight is important because it affects both the handling of the mandrel and its structure. A heavy mandrel requires large cranes, special handling fixtures and high capacity filament winding machines to rotate the large mass. If a mandrel segment weights more than 200 lbs, it can not be safely handled by workers inside a pressure vessel. Also, an incident, such as dropping a segment inside a pressure vessel, can easily damage the fibrous composite structure if the segment is heavy. Even moving a heavy mandrel assembly with an overhead crane entails a far greater risk if the assembly is heavy compared to a lightweight assembly. For instance, bumping into a stationary object is more apt to damage a fibrous composite structure on the mandrel if the assembly weights four times more than it would with a lighter weight mandrel.
Overall stiffness/rigidity and dimensional stability are also important requirements in a mandrel for fabrication of fibrous composite structures. While mandrel stiffness/rigidity are less of a factor when the structure or part being fabricated is processed in equipment which orients the assembly vertically, this approach is not practical when working with large and/or long parts, since it requires high ceilings in the processing bays. Access to the part by workers also is made more difficult due to the height factor. Stiffness and rigidity of the mandrel directly impact the quality of the fibrous composite part fabricated on the mandrel. Any movement of the mandrel while the fibrous composite material is being applied or anytime prior to polymerization (cure) of the resin system causes the fibers to move. This movement displaces the fiber from the desired path which changes the tension of the fibers and in severe cases may cause the fibrous composite material to actually wrinkle. All of these abnormalities significantly reduce the strength and performance of the finished part.
The ideal mandrel for filament winding fibrous composite structures has the following attributes: lightweight, collapsible so it can be removed from the finished part through small ports, rigid and dimensionally stable when assembled, and low life cycle and/or recurring costs. Over the years, numerous attempts have been made to produce the perfect mandrel. While some prior art mandrels have advantages over others, all have significant short-comings.
One prior art mandrel system designed to be easily removed is known as a washout mandrel. This mandrel is formed by packing a mixture of sand and polyvinyl alcohol into a female mold. After a low temperature cure, the polyvinyl alcohol (PVA) acts as a binder, holding the sand granules together. The sand/PVA mandrel can then be removed from the female mold, mated to a winding axis (shaft), and used as is or joined together with other sand/PVA segments to form a larger mandrel. The fibrous composite part is then fabricated, usually by filament winding, and the synthetic resin is cured. Following cure, the sand/PVA mandrel may be removed by spraying hot water into the mandrel which dissolves the PVA allowing the sand to be washed out. Thus, the term washout mandrel. The advantage of this type of mandrel is that it can be removed through a small opening in the finished part. The primary disadvantages are: 1) The weight of about 105 lbs. per cubic foot is very high which limits its use to smaller parts or to vertically-oriented processing to limit bending loads, 2) Fabrication of the mandrel is labor intensive, 3) For large structures the female molds used to fabricate the mandrels are expensive, and 4) The washout process produces a large amount of effluent which must be disposed of as a hazardous waste or treated to remove or reclaim the PVA. Innovations such as using hollow steel cylinders to reduce the volume of sand and PVA required, and replacing the sand with hollow glass microspheres have reduced the weight in some applications.
Segmented collapsible metal mandrels are used in two basic configurations. The first uses a layer of frangible plaster on top of the steel outside contour to form the final outside surface of the mandrel and thus the inside surface or contour of the part fabricated on the mandrel. The support structure of the segmented steel mandrel consists of a winding shaft, which defines the winding axis and is usually a thick walled steel tube that runs from one end of the part to the other and beyond to serve as the attachment point of the mandrel to the filament winding machine. Attached to this winding shaft are radial spoke supports which support the mandrel segments. These mandrel segments are usually made in three pieces which form the cylindrical section and both the forward and aft domes.
The number of segments used is determined by the size of the port they will be removed through after the part is fabricated and the weight which can safely be handled inside the part. The chord length of the mandrel segment can not exceed the diameter of the port from which it will be removed. Ordinary bolt and nut type fasteners are used to attach the mandrel segments to each other and to the winding shaft through the radial spoke supports. A plaster layer is then applied to the outside surface of the mandrel to meet the final contour requirements. In some instances, steel chains and cutting cables are imbedded in the plaster layer to aid in its later removal. This is followed by an oven drying step to remove excess moisture which can adversely affect the polymerization of some epoxy resin systems used in fabricating the part on the surface of the plaster layer.
Following fabrication of the fibrous composite part, the mandrel is removed by disassembling the fasteners securing the segments and supports to the winding shaft. The winding shaft is removed and a worker enters the mandrel and removes the fasteners securing the mandrel segments to one another and any internal support structure. In most cases, this requires persons standing on a temporary platform or with long arms. The mandrel segments are removed, one section at a time. After all the segments are removed, the plaster is broken out and removed. Pulling the chains which are sometimes imbedded in the plaster breaks the plaster free. Plaster removal is an extremely touchy process because the fibrous composite part is easily damaged by impact and the damage is often difficult to detect.
A major advantage of the segmented collapsible metal and plaster mandrel is that it allows for some design changes to the inside contour of the part by adjusting the outer plaster layer of the mandrel. The major disadvantages are its cost, weight and design limitations due to the need for internal access for disassembly.
Segmented collapsible metal mandrels are very similar to the metal and plaster mandrels without the plaster outer layer of the latter. Also, the former have the same attributes as the latter with the exception of not being able to change the outside contour to meet new design requirements. Because of this, metal mandrels are seldom used for development programs where design changes are likely. U.S. Pat. No. 4,448,628 to Stott, entitled Segmental Mandrel for Making Wound Filament Structures, is an example of the segmented collapsible metal mandrel technology.
Mandrels having segments made of composite materials instead of metal have found some limited applications for the fabrication of fibrous composite parts. An extremely important design requirement for a fibrous composite mandrel is that it must be rigid and not deflect, move or change shape under part fabrication and curing loads. Any undesired movement of the mandrel segments during part fabrication can result in deviations of the fibrous composite part from design characteristics and/or wrinkles in the fibers, all of which may have a significant negative effect on the structural integrity of the finished part. Also, tolerances required of the finished part are dependent on an accurate and rigid mandrel.
Composite structures are also fabricated by using an expandable (flexible) male member or mold to compress the fiber and resin composite material against an external female mold. The composite material is then cured while under pressure. For example, U.S. Pat. No. 4,822,272 to Yanase, et al., suggests a segmented male mold made from a super plastic alloy, such as one which contains 78% by weight zinc and 22% by weight aluminum. The alloy is rigid at room temperatures, but when heated to higher temperatures of approximately 180.degree. C. (356.degree. F.) exhibits plastic properties. In this concept, the male mold segments are precast from this superplastic alloy. These segments are then bolted or otherwise fastened together to form the male mold, which includes a central pipe member. After the fibrous composite material is applied to the outside surface of the mandrel, the assembly is heated with circulating hot gas or other fluid which passes through the inside hollow volume of the male mold. As the male mold is heated, the segments become plastic and apply pressure, via the heated pressurized fluid, to the fibrous composite part which is trapped between the male and female molds. After the resin has cured, the female mold segments are unbolted and removed, and the male mold segments are collapsed by application of a negative pressure to the hollow volume inside the male mold, allowing the segments to be removed through a relatively small port.
This approach has several serious drawbacks. The first is that the male mold segments are not reusable because removing the segments by plastic deformation destroys the segments, which greatly increases the recurring costs. Also, use of a zinc and aluminum alloy means that the male mold will not be dimensionally stable due to the high coefficient of thermal expansion (CTE) of these two metals. Zinc has a CTE of 15.2.times.10.sup.-6 in./in./.degree.F. and aluminum has a CTE of 13.0.times.10.sup.-6 in./in./.degree.F. Furthermore, once the segments become plastic, the interior dimensional tolerances of the part being formed are compromised. While this alone may not be a factor in all applications, it clearly limits the application of this concept.
Furthermore, expansion of an internal male mold during cure is not usually a desirable property because of the low strain capabilities of most carbon fibers. While it would be possible to use a resin which cures below the plastic point of the alloy, heating of the male mold to the plastic point for removal could damage the lower temperature resin. Another drawback is how the male mold segments are held in position for assembly prior to being fastened together. Although it appears from the drawings of the Yanase patent that this must be accomplished from the inside, this aspect of the male mold is not addressed.
U.S. Pat. No. 4,597,728 to McGlashen suggests an internal male mold assembly which is expanded by both a mechanical means and a fluid pressure inflatable bladder. McGlashen's male mold is designed to expand in two distinct stages. The first stage uses a rotatable iris to move the mold segments to a first position. Then an inflatable bladder is used to move the segments to a final outermost diameter for pressing the moldable composition against an outer female mold. Thus, the concept is for use in conjunction with an external female mold to apply consolidation pressure during cure of a part captured between the male and female mold assemblies. This approach could not be used to make a structure, such as a pressure vessel, which has limited internal access. Use of the male mold is therefore limited to the fabrication of fibrous composite tubes and similar structures which have at least one unrestricted open end. Also, the weight of McGlashen's mold assemblies would be substantial.
U.S. Pat. No. 4,889,355 to Trimble suggests the use of an expandable solid polyurethane foam as a male mold to aid in forming the seat and chain stay portions of a composite material bicycle frame. The function of this internal molding component is substantially different from that of the mandrel of the present invention. The Trimble male mold is used as an intermediate undersized support and to provide an internal pressing means. The composite material is moved outward by expansion of the polyurethane foam and the composite material is thereby forced against the interior surfaces of an external female mold, causing the composite material to take the form of the female mold cavity during cure of the synthetic resin component of the composite material. Thus, the outer surfaces of the seat and chain stays are determined by the interior surfaces of the female mold. In other words, the male mold of Trimble does not define its own dimensional limits. Instead, these limits are defined by the interior surfaces of the female mold surrounding the mold cavity.
In marked contrast to the male mold assemblies of the prior art, the present invention provides a rigid internal mandrel which defines its own dimensional limits, as well as precise inside dimensions of the part being fabricated on its outer surface. The mandrel of the present invention therefore is designed to rigidly hold its form during part fabrication.
Furthermore, the expandable male mold as suggested in the Trimble patent cannot be removed as a practical matter and therefore remains in the part after its fabrication, whereas the rigid mandrel of the present invention is designed to be removed from the cured composite part with little effort. Thus, the present invention provides a temporary rigid support with fixed and precise dimensions only during fabrication and cure of the part.